1. Field of the Invention
The invention relates to a control system for a hybrid electric vehicle powertrain with an engine and electric motor, and to a control method for managing power distribution when the motor is inactive.
2. Background Art
Parallel hybrid electric vehicles of known design include an internal combustion engine and an electric motor, typically a high voltage induction motor, which establish parallel power flow paths to vehicle traction wheels. The powertrain has two power sources. A first power source is a combination of an engine and a generator subsystem with a planetary gear set for distributing power through separate power flow paths. The second is an electric drive system comprising a motor, a generator and a battery. The battery acts as an energy storage medium for the generator and the motor. The generator, in a parallel hybrid powertrain, is driven by the engine.
A mechanical power flow path is established between the engine and the transmission torque output shaft. The other power flow path is an electrical power flow path, which distributes power from the engine to the generator, the latter providing electrical power to the motor driving the torque output shaft of the transmission through gearing.
When the powertrain is operating with the first power source, the engine power is divided between the two paths by controlling the generator speed, which implies that the engine speed can be decoupled from the vehicle speed. That is, the powertrain can act in a manner similar to a continuously variable transmission, where vehicle speed changes do not depend upon engine speed changes.
The foregoing mode of operation is referred to as a positive split.
The powertrain can act also in a mode of operation that may be referred to as a negative split. In this instance, both the engine and generator input power to the planetary gear, which in turn propels the vehicle. In addition, as in the positive split mode, the engine speed can be decoupled from the vehicle speed. The combination of the motor, the generator and the planetary gear set thus function as an electromechanical, continuously variable transmission.
When a generator brake is activated, the powertrain will act in a so-called parallel mode in which engine power output is transmitted with a fixed gear ratio solely through a mechanical power flow path in the drivetrain.
When the first power source is active, it can provide only forward propulsion since there is no reverse gear. The engine requires either a generator speed control or a generator brake to transmit engine output power to the drivetrain for forward motion. When the second power source is active, the electric motor draws power from the battery and provides propulsion independently of the engine for driving the vehicle forward and in reverse. The generator, at that time, can draw power from the battery and drive against a one way clutch on the engine output shaft to propel the vehicle forward. This mode of operation is called “generator drive.”